Also could front end equallogic arrays. I'm not personally aware(though I have not sought out) any NAS heads that could operate on a back end of iSCSI disks. Given that Equallogic is iSCSI only it sort of limits their options since most "SAN" connectivity is of course fiberchannel.
Exanet can do iSCSI on the back end by contrast since their stuff is fairly agnostic. They already use Chelsio 10GbE NICs, and Chelsio offers 10GbE full iSCSI offload NICs w/linux support(Exanet runs CentOS). Wouldn't take much work to test+certify.
Isilon is a totally different beast to me, with their combined controller+disk units, Exanet is a pure play controller solution, more on the lines of Onstor, NetApp V-series, BlueArc etc.